Advances in transmitter and receiver design, evolution of optical amplification, employment of distributed Raman amplification combined with various dispersion compensation techniques, new encoding and modulation techniques, digital wrapper technology, etc., enabled development of ultra-long reach ULR networks, where an optical signal needs to be regenerated after 3,000 km or more. Since the modern ULR techniques allow optical signals to travel without regeneration over distances greater than the distance between two or more successive nodes, OEO conversion at all intermediate nodes is not necessary in the majority of cases.
However, traditional WDM transport systems use point-to-point connectivity, where all channels are optical-to-electrical-to-optical (OEO) converted at each node. This architecture requires compromises on at least two fronts. Thus, channel performance optimization is possible by either using a full spectrum of channels on each link and constraining the reach of all channels to the reach of the worst performing channel in the system (optimize capacity), or by reducing the channel count of the system by a certain percentage, therefore removing the worst channels, and constraining the reach to that of the worst remaining channel (reduce capacity to optimize reach).
As a result, the advantages of the ULR cannot be fully exploited in point-to-point network architectures.
There is a trend towards a new generation of optical networks that takes full advantage of the modern ULR techniques to obtain important cost savings by eliminating the equipment used for the unnecessary OEO conversion. An agile photonic network switches a channel in optical format, while automatically regenerating the respective channel only when necessary. Reduction of the regeneration sites dramatically reduces the node complexity, and consequently the network cost. In addition, the new connection oriented architecture enables service automation, so that the user can automatically establish an end-to-end connection at a push of a button. This again results in important cost and time savings, in that it practically eliminates the manual network engineering required by the current optical networks. Also, this new architecture has, among numerous other advantages, the ability to treat each connection differently, so as to provide the respective user with an individualized class of service, with the corresponding revenue differentiation.
The parent US patent application Ser. No. 09/909,265 describes an automatic wavelength routing and switching mechanism for an agile photonic network. In this type of network, regenerators and wavelengths are two of the most important network resources; methods to economically use these resources are crucial to cost reduction and operational efficiency. These resources are allocated to a connection according to certain rules, which are dictated by the class of service for the respective connection, and by the current network connectivity and loading. New rules may be readily added, or the existing rules may be amended for a certain desired result.
As described in the above-referenced U.S. patent application Ser. No. 10/017,833, due to the transmission medium (fiber) intrinsic characteristics, the channel reach is a natural function of the channel wavelength. In addition, current technologies being used in WDM systems, such as Raman and/or EDFA optical amplification and slope matched dispersion compensation, further differentiate the performance of a channel according to its wavelength and to the wavelengths of the co-propagating channels. Experiments show that the ratio between the reach of the “best” and “worst” performing wavelengths can be more than 2:1 for the same launch power. Therefore, the wavelength assignment may be optimized if the network management takes into account the individual wavelength performance.
U.S. patent application Ser. No. 10/128,092 (Kotikalapudi et al.) describes the basic rules for wavelength assignment, which take into account the wavelength performance and current wavelength assignment. The wavelengths are organized in binning tables, or bins, based on their performance. Once a bin corresponding to the length of the optical path under consideration is identified in the binning table, the wavelength for that path is selected based on path length alone, or based on the length and one or more additional parameters. Such additional parameters are for example the wavelength spacing, the load on the respective optical path, the fiber type, network-wide wavelength utilization (fragmentation), etc. The optical path performance is estimated for the selected wavelength, and the search continues if the estimated path performance is not satisfactory. Several available wavelengths are identified and the wavelength that is most used is selected and assigned to the respective optical path.
It has been noted that the position in the spectrum of a wavelength selected for a new connection is important. More precisely, it is important that the channels traveling along any optical path are spaced apart so as to minimize signal-to-signal coupling due to stimulated Raman scattering (SRS). SRS is a non-linear phenomenon, which results in migration of power from lower wavelength channels to higher wavelength channels. Thus, SRS transfers the power of one or more Raman pumps of a certain wavelength to the channels of the WDM signal passing through the pumped fiber. On the other hand, SRS also redistributes the optical power between the channels of the WDM signal. For example, a large number of channels with shorter wavelengths can significantly amplify the longer wavelength channels. Also, a close spacing between the channels creates an amplification effect that results in the spectrum of the WDM signal being undesirable tilted, the tilt increasing with the signal power.